This invention pertains to methods and apparatuses for the capacitive radio frequency (RF) dielectric heating of foods.
A variety of different methods are available for the thermal processing of various materials. Heat is supplied by hot water, steam, resistive heating elements, burners, torches, ovens, electrical conduction (ohmic heating), induction heating (magnetic), capacitive heating (dielectric), and electromagnetic radiative heating (resonant ovens, cavities or chambers) and many other heating methods. Applications include sterilization, pasteurization, thawing, melting, curing, drying, bonding (e.g. laminates), welding, brazing, heating for chemical reactions, and many others. Heated materials include ceramics, rubber, plastics (and other polymers), composites, metals, soils, wood and many types of biological materials including food.
An important application of heating technologies is in the area of the pasteurization and sterilization of foods, particularly foods in large-dimensioned packages. Food safety and quality is becoming an increasingly important topic with the many incidents where people have become sick or died due to unkilled microbial populations in food. For example, alfalfa and radish seeds are raw agricultural commodities that can become contaminated with organic material that harbor pathogens such as Salmonella or E. coli O157:H7 during growing and harvest. Seed processing and storage procedures are aimed at reducing varietal contamination of seeds through the elimination of weed seeds and foreign matter. Such seed cleaning and certification programs insure varietal purity, but provide no means of food safety intervention for seeds destined for sprouting and consumption as food. As a result, there are increasing reports of microbial outbreaks in sprouted seed products such as radish and alfalfa sprouts. Human salmonellosis (due to Salmonella bacteria) and outbreaks of E. coli O157:H7 have been associated with the consumption of alfalfa and radish sprouts in several countries. Alfalfa and radish sprouts, a definitive highly nutritious and perceived healthy food, have been implicated in multi-site outbreaks of foodborne illnesses. Seeds were linked to about 150 confirmed cases of salmonellosis in Oregon and British Columbia in 1996. Also in 1996, radish sprouts were associated with Japan's largest recorded outbreak of E. coli O157:H7 infection with an estimated 11,000 cases that led to eleven deaths. In June and July 1997, simultaneous outbreaks of E. coli O157:H7 infections in Michigan and Virginia were independently associated with eating alfalfa sprouts grown from the same seed lot. A total of 60 people with E. coli O157:H7 infection were reported to the Michigan Department of Community Health and 48 cases reported to the Virginia Department of Health. Recently, the California Department of Health Services identified six cases of E. coli O157:NM with illness onsets from June 16 through Jun. 27, 1998, caused by eating an alfalfa-clover sprout mixture.
The lack of standardization in required heating time/temperature relationships to insure food product safety is also attracting more focus. In addition, food quality or taste/texture issues are important in our selective consumer oriented society. Therefore there is a need for a heating technology that will achieve the desired microbial kill rates uniformly over that whole food product in a reasonable amount of time without altering or degrading the overall quality of the food.
In the seafood industry, for example, existing heating technologies for the pasteurization of seafoods employ either hot water or steam, sometimes in combination with gas preheaters. These technologies have several limitations including reliance on thermal conduction from the product surface (resulting in non-uniform heating), slow heating rates (especially in the product center), large floor space requirements, poor overall energy efficiency, generation of large amounts of waste water and limitations on the product geometry (i.e., need to be thin or flat).
Capacitive RF dielectric heating is used in several industries. They include the drying of various wood and sawdust products in the timber industry, preheating and final drying of paper, drying of textiles, drying of glass fibers and spools, drying water-based glues in the paper-cardboard industry, drying pharmaceutical products, welding plastics, sealing, preheating plastics prior to forming, firing foundry cores in casting, polymerization of fiber panels, gluing of woods such as laminated plywood, printing and marking in the textile, leatherware and shoe industries, melting honey, heating rubber prior to vulcanization, welding glass formed sections, bonding multi-layer glass products, drying of powders, drying leathers and hides, curing of epoxy, curing of plastisol, curing of brake linings, impregnating resins, thermosetting adhesives, curing hardboard and particle board, and many other applications.
The use of capacitive (RF) dielectric heating methods for the pasteurization and sterilization of foods offer several advantages over non-electromagnetic heating methods. These include rapid heating, near independence of the thermal conductivity of the medium (i.e. heat internal portions of medium directly), high energy efficiency, good heating even in the absence of DC electrical conductivity, high energy densities, reduced production floor space, and easy adaptation to automated production batch and/or continuous flow processing.
Another application of this technology is in the thawing of frozen foods. Common thawing applications again rely on the thermal conduction of heat from the surface to the interior to provide thawing. Due to freshness and product quality constraints thawing often is done by immersion in water baths that are only slightly above freezing themselves or in refrigerators set to slightly above freezing (e.g. 35-40.degree. F.). Thawing times are often very long and can be overnight. With capacitive heating teclmologies, that heat over the entire volume uniformly, thawing can be performed much more rapidly.
Capacitive (RF) dielectric heating differs from higher frequency electromagnetic radiative dielectric heating (e.g. microwave ovens) in that with capacitive heating the wavelength of the chosen frequency is large compared to the dimensions of the sample being heated whereas with electromagnetic radiative heating the wavelength is comparable or even small compared to the dimensions of the sample being heated. An example of capacitive heating is two large parallel electrodes placed on opposite sides of a wood sample with an AC displacement current flowing through it to heat and dry the wood. An example of electromagnetic radiative heating is a metal chamber with resonant electromagnetic standing wave modes such as a microwave oven. Capacitive heating also differs from lower frequency ohmic heating in that capacitive heating depends on dielectric losses and ohmic heating relies on direct ohmic conduction losses in a medium and requires the electrodes to contact the medium directly (i.e. can not penetrate a plastic package or air gap).
Capacitive (RF) dielectric heating methods offer advantages over other electromagnetic heating methods. For example, capacitive (RF) dielectric heating methods offer more uniform heating over the sample geometry than higher frequency radiative dielectric heating methods (e.g. microwave ovens) due to superior or deeper wave penetration into the sample as well as simple uniform field patterns (as opposed to the complex non-uniform standing wave patterns in a microwave oven). In addition capacitive (RF) dielectric heating methods operate at frequencies low enough to use standard power grid tubes that are both lower cost (for a given power level) as well as allow for generally much higher power generation levels than microwave tubes. Capacitive (RF) dielectric heating methods also offer advantages over low frequency ohmic heating. These include the ability to heat a medium that is enclosed inside an insulating plastic package and perhaps surrounded by an air or de-ionized water barrier (i.e. the electrodes do not have to contact the media directly). The performance of capacitive heating is therefore also less dependent on the product making a smooth contact to the electrodes. Capacitive (RF) dielectric heating methods are not dependent on the presence of DC electrical conductivity and can heat insulators as long as they contain polar dielectric molecules that can partially rotate and create dielectric losses. A typical existing design for a capacitive dielectric heating system is described in Orfeuil, M. 1987. Electric Process Heating: Technologies/Equipment/Applications. Columbus: Battelle Press.
Capacitive (RF) heating devices have been used in the food industry, but the reported energy efficiency has been low and heating has not always been uniform. Proctor Strayfield has developed a magnatube pasteurization system (Koral, A. L., 1990. Proctor-Strayfield Magnatube Radio Frequency Tube Heating System. Proctor Strayfield, A Division of Proctor & Schwartz, Inc.) that has been demonstrated to be successful in the cooking/sterilization of scrambled eggs as well as in the creation of a "skinless" meatloaf from a pumped slurry using a vertical tube system. Houben et. al of the Netherlands (Houben, J., Schoenmakers, L., van Putten, E., van Roon, P. and Krol, B. 1991. Radio-frequency pasteurization of sausage emulsions as a continuous process. J. Microwave Power & Electromagnetic Energy. 26(4): 202-205.) in 1991 showed that sausage emulsions could be successfully pasteurized using RF heating. Bengtsson et al of Sweden (Bengtsson, N. E., and W. Green. 1970. Radio-Frequency Pasteurization of Cured Hams. Journal of Food Science. V35: 681-687) in 1970 demonstrated that cured hams could be pasteurized successfully by RF heating. RF heating feasibility experiments were conducted on packaged and unpackaged surimi seafood samples at a test facility of PSC, Inc. of Cleveland, Ohio (Kolbe, Kolbe, E. 1996. Heating of packaged surimi seafoods in a commercial RF oven. Unpublished information. OSU Dept. of Bioresource Engineering.). The test system was a high power single-frequency capacitive heater set at 18 MHz. Tests on samples placed between parallel electrodes showed that when properly oriented, surimi seafoods could be heated to pasteurization temperatures (85.degree. C.) in less than 10 minutes. The results also showed, however, that packaging can be a complicating factor. For example, small amounts of food trapped in the packaging seams can cause rapid local heating and burning.
Some prior work in the area of dielectric heating has been conducted on seed germination enhancement effects. The possibility of utilizing dielectric energy for stimulating or improving the germination and growth of seeds and for controlling insects has been variously considered for the last forty years (Nelson, S. O. and Walker, E. R. 1961, "Effects of radio-frequency electrical seed treatment," Agricul. Eng. 42(12): 688-691; Nelson, S. O. 1976, "Use of microwave and lower power frequency RF energy for improving alfalfa seed germination," J. Microwave Power 12(1):67-72; Nelson, S. O. 1996, "Review and assessment of radio-frequency and microwave energy for stroed-grain insect control," Transactions of the ASAE. 39(4):1475-1484.). Reported effects ranged from accelerated germination and early growth and the killing of fusarium spores to early flowering and high yields of plants grown from treated seeds. Nelson and Walker (1961) reported that brief exposure of alfalfa containing considerable hard seed shells to electrical fields has been highly successful in reducing the head-seed percentages and producing a corresponding increase in normal seedling germination. Also, benefits from electric treatment have been shown in alfalfa seeds for up to 21 years in storage with no evidence of any short or long term detrimental effects (Nelson, 1961, 1976). Nelson (1976) found that the moisture content of seeds at the time of treatment influenced the degree of response. Generally seeds of lower moisture content responded more favorably to treatment than high moisture content seeds. The final temperature of seeds treated at any given moisture content seemed to be a good indicator of the degree of favorable response. Some work has been done using higher frequency microwave heating for the treatment of seeds. Cavalcante et al. (Cavalcante, M. J. B. and Muchovej, J. J. 1993, "Microwave irradiation of seeds and selected fungal spores," Seed Sci. & Technol. 21:247-253) investigated the use of microwave irradiation on seeds and its effects on the control of selected fungal spores.
Some work has been done to characterize the dielectric properties of food and packaging materials. There is preliminary data at lower frequencies for polymers to show temperature dependent Debye resonance effects (Malik, T. M., R. E. Prud' Homme. 1984. Dielectric Properties of Poly(.alpha.-Methyl-.alpha.-N-Propyl-.beta.-Propiolactone)/Poly(Vinyl Chloride) Blends. Polymer Engineering and Science. v24, n2 p144-152; Scarpa, P. C. N., Svatik, A. and Das-Gupta, D. K. 1996. Dielectric spectroscopy of polyethylene in the frequency range of 10.sup.-5 Hz to 10.sup.6 Hz. Polymer Eng. & Sci. 36(8): 1072-1080). And, for food in the medium frequency ranges, limited tabulated data exists (Von Hippel, A. R., 1954. Dielectric Materials and Applications. MIT Press; Kent, M. 1987. Electrical and Dielectric properties of food materials. Science and Technology Publishers, England; Mudgett, R. E. 1985. Electrical Properties of Foods. In Microwaves in the Food Processing Industry, R. V. Decareau (Ed.). New York: Academic Press; Pethig, R. 1979. Dielectric and Electronic Properties of Biological Materials. Chichester: John Wiley & Sons, Inc.; Tinga, W. R. and S. O. Nelson. 1973. Dielectric Properties of Materials for Microwave Processing-Tabulated. J. of Microwave Power. 8:1-65; Tran, V. N. and Stuchly, S. S. 1987. Dielectric properties of beef, beef liver, chicken and salmon at frequencies from 100 to 2500 MHz. J. Microwave Power. 29-33). Most data for food has been collected at higher frequencies (&gt;100 MHz) and tied closely to the dielectric behavior of the water in the medium, for applications toward microwave ovens.
A specific disadvantage of capacitive (RF) dielectric heating methods include the potential for thermal runaway or hot spots in a heterogeneous medium since the dielectric losses are often strong functions of temperature (e.g., small pockets of a lossy dielectric food material, for example a small thermal mass trapped in the seams of a package, may heat rapidly and could burn itself and the melt the package). Another disadvantage of capacitive heating is the potential for dielectric breakdown (arcing) if the electric field strengths are too high across the sample (necessitating compensating air gaps or a thicker sample).
The use of edible films to extend shelf life of food products and protect them from harmful environmental effects has been emphasized in the recent years. Interests and research activities in edible films have been especially intense over the past ten years. Edible films are very promising systems for the future improvement of food quality and preservation during processes and storage. Indeed, edible films can be used where plastic packaging cannot be applied. For example, they can separa te seve ral compartments within a food. Although edible films are not meant to totally replace synthetic films, they do have the potential to reduce packaging and to limit moisture, aroma, and lipid migration between food components where traditional packaging cannot function.
An edible film is defined as a thin layer of one or more edible materials formed on a food as a coating or plaed (pre-formed) on or between food components. Most edible films are natural polymers obtained from agricultural products such as animal and vegetable proteins, gums, and lipid and are perfectly biodegradable and usually water soluble. The general materials that are used to manufacture edible films are cellulose ethers, starch, corn zein, wheat gluten, soy proteins and milk protein. Examples include methyl cellulose (MC), hydroxypropyl cellulose (HPC), Sodium and Calcium Caseinates (SC or CC), and whey protein concentrates (WPC).
The performance of edible packaging is comparable to that of traditional synthetic polymer films with respect to mechanical strength, barrier properties, and compatibility. Applications of edible packaging include its use in inhibiting migration of moisture, oxygen, carbon dioxide, aromas, and lipids, etc. within composite foods; carrying food ingredients (e.g, antioxidants, antimicrobials, flavor); and/or improving mechanical integrity or handling characteristics of the foods.
Moisture transport through polymer films is influenced by several polymer properties including chemical structure, method of polymer preparation, polymer processing condition, free volume, density, crystallinity, polarity, tacticity, crosslinking and grafting, orientation, presence of additives, and use of polymer blends. An increase in crystallinity, density, orientation, molecular weight or crosslinking results in decreased permeability of edible films.
Although capacitive (RF) dielectric heating systems have been used for heating foods in the past, there remains a need for improved methods and apparatuses to rapidly, efficiently and uniformly heat food products or parts of food products.
It has now been discovered that certain capacitive (RF) dielectric heating devices and/or methods can be used to rapidly, efficiently and/or uniformly heat foods, including unsprouted seeds and sprouts, for pasteurization, sterilization and/or thawing.
The performance of capacitive (RF) dielectric heating methods is highly dependent on the dielectric properties of the materials being heated. Optimal heating is obtained by matching the frequency of the RF energy input to the dielectric properties of particular food products to be heated. For example, the frequency of the AC displacement current can be chosen to heat muscle tissues in a food product, instead of just the water that is heated in a microwave heating process. Potential thermal runaway problems are thus avoided.
The energy efficiency and/or heating rate are maximized at or near the location in frequency of a "Debye resonance" of the material to be heated. Thus, in the heating system of the present invention, the RF signal frequency is tuned to the optimal Debye frequency or frequencies of a component or components of the food product to be heated. Multiple Debye resonances may occur in a composite material. So, multiple composite frequency groups can be applied to handle the several Debye resonances. Also, the RF signal frequencies can be varied with temperature to track Debye frequency shifts with changes in temperature.
Characterization of dielectric properties vs. frequency and temperature is thus a first step to the design of a capacitive (RF) dielectric heating system to pasteurize, sterilize or thaw various foods by the method of the present invention. It is usually desired to heat food without any noticeable degradation in quality, texture or taste. Thus, to aid in the selection of appropriate operating conditions, food samples are studied to assess the effects of RF energy on key properties of the food samples at various frequencies and temperatures. The results of these studies influence the design of capacitive dielectric heating systems according to the present invention.
In addition, a capacitive dielectric heater may be called on to heat a food product contained in packaging materials. Some packaging materials will degrade if overheated, whereas other packaging materials can be heated intentionally to transfer heat energy to a food product contained in the packaging. Therefore the characterization of the dielectric properties of packaging materials and the effects of RF energy on those materials may be important in choosing the proper packaging materials and/or operating frequencies.
An electromagnetic/heat transfer mathematical model can be used to predict the dielectric heating characteristics of various foods and packaging materials. Such a model may involve 2-D and/or 3-D mathematical modeling programs as well as finite element methodologies to model composite materials. Best results are achieved with a model that integrates both electromagnetic and heat transfer principles.
To supply the AC displacement current at a needed frequency, variable components in a tunable RF signal generator circuit and associated matching networks are actively tuned to change frequency, or tuned automatically, or switched with a control system. Therefore a software control system is also provided to set up the frequency profile. A variable frequency synthesizer and a broadband power amplifier and associated matching systems and electrodes are useful components of a medium energy version of such a capacitive dielectric heating system according to the present invention. Temperature monitoring of the heated medium using thermal sensors or infrared scanners is conducted and data fed back into the control system and the generator's frequency groups swept accordingly to track the Debye shifts with temperature.
With the methods and apparatuses of the present invention it is possible to avoid the potential disadvantages of capacitive (RF) dielectric heating methods mentioned above. In particular, the potential limitations are addressed by providing frequency control to match Debye resonances of the dominant constituents of the medium, track them with temperature, control field strengths and optimize product geometries to prevent arcing. Packaging methodologies can be used to keep food out of seams to avoid heating the seams. And, the electrode system may be constructed to lower the field strengths on the packaging seams. To prevent or reduce the risk of thermal runaway, a gridded electrode system can be used with an infrared scanner to monitor the entire body of a food product being heated. In response to signals from the scanner, individual components of the food product can be independently heated by adjusting local field strengths or by switching some portions of the grid off in different duty cycles to prevent "hot spots".
Capacitive (RF) dielectric heating systems according to the present invention offer numerous advantages over hot water and steam heating (for pasteurization or sterilization) and cold water thawing technologies and other heating technologies.
By certain processes of the present invention, it is possible to obtain uniform heating over the entire volume of a food product. This technology heats the polar dielectric molecules evenly and directly over the entire volume of the sample as opposed to hot water and steam heating or cold water thawing technologies that rely on thermal conduction from the surface of the medium. Capacitive (RF) dielectric heating offers more uniformity than higher frequency microwave ovens due to the lower RF frequency range which, for the purpose of this disclosure, is defined as being about 300KHz to 300 MHz, and preferably 1 MHz-100 MHz, allowing for deeper penetration into saline media as well as the fact that the media is small compared with a wavelength and so capacitive heating does not rely on the complex wave propagation and reflection modes required in a microwave oven. The field patterns are generally simple 2-D patterns between parallel plates again resulting in more uniformity in heating.
Heating to pasteurization and sterilization temperatures can be rapid, resulting in lower degradation to food quality (e.g. protease enzyme in seafoods inactivated). The rapid heating capability is due to the same uniform heating advantage described above and by the matching of generator frequency or composite of frequencies to the Debye resonance frequency groups of the various food media and/or packaging, and tracking those Debye resonance frequency groups with temperature. Power control capability of the generator/heating system allows for the ability to set heating rates to optimize heating processes. Production floor space may also be reduced due to increased throughput rates.
Higher overall energy efficiency is obtained by matching the generator frequency or composite of frequencies of the RF signal to the Debye resonance frequency groups of the various food media and tracking those resonances with temperature resulting in a shorter heating time per unit volume for a given energy input.
Complete control of the heating process is achieved by the selective heating of various constituents of the food medium and/or packaging material. Protein molecules contain peptide chains with amino-acid side groups that often are polar. In addition various hydrated interfaces (bound water) of complex tissue molecules can also be polar. This technology can be set up to target the Debye resonances of those constituents of food for which heating is desired and avoid the Debye resonances of other constituents (e.g. packaging materials) of which heating is not desired by setting the generator frequency or frequency groups of the RF signal to target the appropriate Debye resonances and track them with temperature and avoid other Debye resonances.
Capacitive (RF) dielectric heating will heat a food sample even when packaged in a thermal and electrical insulator as well as if the sample is thick and unusually shaped due to the fact that the internal polar molecules are being heated and again therefore no reliance on thermal conduction from the surface. Hot water and steam technologies require flat geometries with very thin packaging or insulating layers.
Capacitive (RF) dielectric heating of food is a clean process with no generation of wastewater.
Heating rates can be increased by the matching of the generator frequency or composite of frequencies of the RF signal to the Debye resonance frequency groups of the various heated media and tracking those Debye resonance frequency groups with temperature.
Overall energy efficiency is improved due again to the matching of the generator frequency or composite of frequencies to the Debye resonance frequency groups of the various heated media and tracking those Debye resonance frequency groups with temperature. Efficiency is also improved by selective heating of the various individual constituents of a medium (e.g. glue between layers of plastic packaging laminates) by targeting the Debye resonance profiles of those constituents and setting up the generator to excite them and track them with temperature.
One appropriate use of capacitive (RF) dielectric heating is for the sterilization and/or pasteurization of seeds packaged in edible films or alternatively standard polymer films. The technology may also be useful to improve or enhance the germination rates of seeds through the pretreatment of seeds prior to planting and sprouting with capacitive (RF) dielectric heating. The warming and softening of seed shells as well as the control or reduction/elimination of germination inhibiting microorganisms are the two primary methods of improving germination rates when using capacitive (RF) heating technology as a pre-germination seed treatment process.